Do not create move paths that do not need dropping.

Author: cjgillot

compiler/rustc_borrowck/src/lib.rs

@@ -33,7 +33,7 @@ use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::*;
 use rustc_middle::query::Providers;
 use rustc_middle::traits::DefiningAnchor;
-use rustc_middle::ty::{self, Ty, CapturedPlace, ParamEnv, RegionVid, TyCtxt};
+use rustc_middle::ty::{self, CapturedPlace, ParamEnv, RegionVid, Ty, TyCtxt};
 use rustc_session::lint::builtin::UNUSED_MUT;
 use rustc_span::{Span, Symbol};
 use rustc_target::abi::FieldIdx;

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -55,6 +55,7 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
         let def_id = body.source.def_id();
         let param_env = tcx.param_env_reveal_all_normalized(def_id);
         let move_data = MoveData::gather_moves(&body, tcx, param_env, |ty| match ty.kind() {
+            _ if !ty.needs_drop(tcx, param_env) => PathFilter::Skip,
             ty::Ref(..) | ty::RawPtr(..) | ty::Slice(_) => PathFilter::Leaf,
             ty::Adt(adt, _) => {
                 if adt.has_dtor(tcx) && !adt.is_box() {
@@ -371,7 +372,16 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
             let terminator = data.terminator();
 
             match terminator.kind {
-                TerminatorKind::Drop { place, target, unwind, replace } => {
+                TerminatorKind::Drop { place, target, unwind, replace: _ } => {
+                    if !place
+                        .ty(&self.body.local_decls, self.tcx)
+                        .ty
+                        .needs_drop(self.tcx, self.env.param_env)
+                    {
+                        self.patch.patch_terminator(bb, TerminatorKind::Goto { target });
+                        continue;
+                    }
+
                     self.init_data.seek_before(loc);
                     match self.move_data().rev_lookup.find(place.as_ref()) {
                         LookupResult::Exact(path) => {
@@ -404,18 +414,10 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
                                 bb,
                             )
                         }
-                        LookupResult::Parent(..) => {
-                            if !replace {
-                                self.tcx.sess.delay_span_bug(
-                                    terminator.source_info.span,
-                                    format!("drop of untracked value {bb:?}"),
-                                );
-                            }
-                            // A drop and replace behind a pointer/array/whatever.
-                            // The borrow checker requires that these locations are initialized before the assignment,
-                            // so we just leave an unconditional drop.
-                            assert!(!data.is_cleanup);
-                        }
+                        // A drop and replace behind a pointer/array/whatever.
+                        // The borrow checker requires that these locations are initialized before the assignment,
+                        // so we just leave an unconditional drop.
+                        LookupResult::Parent(..) => {}
                     }
                 }
                 _ => continue,